Interesting. Because the bar bends the torso a bit more forward, I've noticed that it forces people to engage their posterior chain. That's typically a good thing. I guess this is why we have different styles of squats though. I have a big torso and long femur. Low bar is infinitely easier and puts less torque on my spine. The first time I squatted low bar I gained 10 pound on my 5RM and felt safer. This just goes to show that people should try out different styles to see which works best for them.
Low bar means less of a moment when you squat as oppose to high bar for the same amount of lean forwards. But th is depends very much on how much you lean forwards though.
Do a front squat. THIS WILL QUICKLY TELL YOU if you can have the mobility to back squat properly.
Theres no real reason why you can't squat properly with and without weight. I'm willing to bet your not ATG with weight and that you can't go ATG without weights. You may go lower with weights as it helps to push you down if your inflexible but this would not mean you could not back squat without any weights.
It's a mobility issue.
You can squat with a weight out in front as it helps counter balance you as your knees aren't going much over your toes. The further they go out over the easier it is to 'sit down inbetween your feet' that should be pointing at 10 and 2 o clock. High bar Back squat you typically want to sit 'down' and not 'backwards' in a more PowerLifting orientated squat. PowerLifting you want to get to parallel and get back up, typically wider feet will help you get to parallel. It's hard to get to parallel with a high bar narrow (shoulder width or a touch wider) back squat.
Put some 2.5kg plates under your heels. That will help you squat deeper due to it cutting your range of movement. It will help you stay more upright and not 'fall backwards'.
The best way to learn to back squat is to FRONT SQUAT. If you can't do it that means your back squat is not going to be text book. Your lean forwards will mean you drop the bar in the front squat. But when you back squat you'll probably just lean forwards or not go all the way down.
Work on your mobility and you will be able to do it!
Koing